Electrical strain gauge

ABSTRACT

An electrical strain gauge is provided in which the strain-responsive circuit element is formed from a thin film deposit on a surface of a metal substrate on which an insulating film has already been deposited. Connections are made directly to the circuit element by the ends of connecter leads that are bonded into insulating plugs in apertures in the substrate. The plugs and the insulating film are glasses to which the substrate and the circuit element are bonded. A further glass layer encapsulates the circuit element. As a result a very robust form of gauge is provided that can be used in high temperatures and in chemically active environments.

BACKGROUND OF THE INVENTION

This invention relates to electrical strain gauges.

In the design and use of electrical strain gauges, it is important toensure that a stable response can be obtained from the strain-responsiveelectrical element that is directly or indirectly attached to the memberthe strain of which is to be measured, if accurate measurements are tobe made.

For this purpose, strain-responsive elements deposited on a suitablesubstrate by thin-film techniques are found to be superior to elementsetched from a plated-on foil. These techniques can provide a firmlyadherent layer on a substrate, and so provide a strain-responsiveelement that is free of problems of long-term creep and hysterisisoccuring in the use of foils bonded onto the substrate by a plasticsadhesive (e.g. epoxy resin), as well as avoiding the temperaturelimitations imposed by such adhesives.

Known thin-film techniques comprise methods of forming solid layers bycondensation from the vapour phase, including vacuum depositionprocesses, e.g. sputtering and chemical vapour deposition. Such layersare usually deposited with a thickness of less than 2 microns, althoughgreater thicknesses are possible, and the resultant thin-film will havecharacteristics typical of a discontinuous layer or of a bulk materialdepending upon the thickness. The term "thin-film" as used herein isintended to refer to deposits produced by thin-film techniques andcapable of providing a flow path for an electrical current.

Strain gauges incorporating such strain-responsive elements typicallycomprise a layer of glass or other insulating material deposited on asurface of the member to be monitored, e.g. by sputtering, as aninsulating layer, the underlying member commonly being metallic, and athin-film strain-responsive layer deposited on the insulating layer andetched to form a resistive circuit element. The connection of theelement to an external measurement circuit is made by wire bonds using aprinted circuit board also attached to the surface of the memberadjacent to the resistive element for the junction of the wire bondswith the lead-outs to the measuring circuit. However, these wire bondsare relatively fragile in use and may be prone to chemical attack.

Another aspect of strain gauge measurement lies in that it is oftendesirable to operate in hostile environments. It is well known to covera strain gauge element with organic encapsulating materials which serveto protect the element from dirt and moisture, but this measure is oflittle use at high temperatures. Moreover, the adherence of a plated-outstrain gauge element will be lost if it is subjected to temperaturesabove the limits for the adhesive bonding materials and substrates usedto attach the element to the member being monitored, and even though athin-film element may itself be resistant to high temperatures theprinted circuit board by which it is connected to the measurementcircuit is not.

As a result, when the critical parameter to be monitored is the strainat a position such as the inner wall of a container, e.g. a pressurevessel, filled with heated fluid, it may not be possible to site astrain gauge on the surface in question. The measurement must then bemade indirectly from another region of the container walls, with theresult that there is a loss of accuracy and of stability in the strainsignals generated.

Finally it may be mentioned that a form of packaging for the solid statedevices is known from U.S. Pat. No. 3,444,619 to improve the durabilityof the device, in which the device is attached by an adhesive such assolder to a base element in apertures of which electrical leads havebeen fixed in insulating glass inserts, the wire bonds of the devicebeing connected to the exposed ends of the inserted leads, and a metalcover then being soldered or welded to the base element to enclose thesolid state device and the wire bond connections to the leads. Such anarrangement if employed for a strain gauge would have little value,however, in meeting the problems outlined above.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided anelectrical strain gauge comprising a strain-responsive circuit elementformed from a thin-film deposit bonded onto a substrate, connections tosaid circuit element being provided by leads projecting through thesubstrate to its surface to end flush with that surface and in contactwith the deposited film, and a protective or passivating materialcovering the thin-film element.

More particularly, there may be provided an electrical strain gaugecomprising a metallic substrate carrying a strain-responsive electricalcircuit element formed by a thin-film deposit overlying a non-conductivelayer insulating the circuit element from the metallic substrate, thenon-conductive layer being bonded both to the substrate and to thecircuit element, conducting leads projecting through the substrate toterminate flush with the surface of said substrate on which thenon-conductive layer lies, said layer being discontinuous at thelocations of said leads to permit electrical contact between said leadsand the circuit element, and a protective or passivating layerencapsulating said circuit element.

In this manner, the connecting leads for an external measurement circuitcan be put in direct contact with the thin-film element generatingstrain-responsive signals and without the interposition of wire bonds,the leads being inserted and fixed in the substrate or body of thestrain gauge before the deposition of the circuit element material.Moreover, the leads can easily be arranged to emerge from the substrateon the opposite face to that carrying the thin-film element; if theprotective layer on the element is itself heat resistant, e.g.comprising glasses, the circuit element can operate on a face of amember exposed to high temperatures and chemically active materialswithout risk of damage to the strain gauge.

Preferably a deposit of conductive material is provided over the circuitelement in the regions of the leads before protective or passivatingmaterial is applied, in order to increase the conductivity of saidregions and shunt these portions of the circuit element.

According to a further aspect of the invention, there is provided amethod of manufacturing an electrical strain gauge in which

(i) conductor leads are secured in apertures passing through a metallicsubstrate with insulating material surrounding the leads in thesubstrate to isolate them therefrom,

(ii) a surface of the substrate to which said leads extend is polishedsmooth with the ends of the leads made flush with said surface,

(iii) a deposit of an insulating material in the form of a film bondingwith the metallic substrate is laid on said surface,

(iv) with said ends of the leads exposed, a thin-film deposit for astrain-sensitive electrical circuit element is applied to bond with theinsulating film and to make electrical contact with the leads, and

(v) a protective or passivating layer is applied over the circuitelement.

The initial insulating layer and the overlying protective layer may beeach composed of glasses, thereby offering a high tolerance to elevatedtemperatures and corrosion substances, and they may be applied bysputtering. Glasses may also be used in the form of plugs in saidapertures to bond the conductor leads in place and isolate them from themetallic substrate.

The resistive material may be deposited in the final form of the circuitelement, possibly by the use of masks, or it may be etched to therequired pattern after deposition. A layer of conductive material may beapplied to discrete portions of the circuit element in the immediateareas of said lead ends, in order to increase the conductivity in saidareas, but it will not usually be necessary to apply the conductivematerial as a thin-film deposit. If etching methods are used to form thecircuit element and said conductive areas, the different materials canbe etched in separate baths sequentially or a common bath with selectiveetchants may be used for acting on the circuit element and itsassociated conductive areas simultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail by way of example, withreference to the accompanying schematic drawings.

FIGS. 1 and 2 are detail sectional views of a part of a strain gaugeaccording to the invention and a broken-away plan view of the same partof the gauge respectively, illustrating the connection of a conductorlead to the strain-sensitive element, and

FIGS. 3 to 5 are outline illustrations of different forms of straingauge according to the invention, having the features shown in FIGS. 1and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIGS. 1 and 2 can be seen a strain-responsive electrical resistanceelement 2, e.g. of Nicrome, formed as a thin-film deposit on asupporting body or substrate 4. The element is in contact with aconductor 6, e.g. of Kovar (Trade Mark), a metal alloy (e.g., Fe 53.8,Ni 29, Co 17, Mn 0.2%) that provides a lead-out through the thickness ofthe substrate to connect the element with a strain-measurement circuit(not shown). The substrate 4 is metallic, e.g. of stainless steel, andis insulated from the conductor by a glass plug 8 bonded to both.Underlying the resistance element 2 is a glass layer 10 insulating italso from the metallic substrate, but this layer is interrupted at theconductor 6, to establish contact between the conductor and theresistance element 2 at that point, while overlapping the plug 8 to keepthe element 2 isolated from the substrate. Over the area of contactbetween the element 2 and the conductor 6, and the adjacent region, anelectrically conductive layer 12, e.g. of copper or aluminium, is putdown on top of the thin-film deposit 2, this layer possibly beingsomewhat thicker than the deposit 2, and both are encapsulated by aglass passivating layer 14.

The strain gauge will have a plurality of conductors 6 making contactwith spaced regions of the thin-film deposit 2 and at each such regionthere may be a local deposit of the conductive material layer 12 so asto control the area of the thin-film deposit from which measurements aremade and thereby increase the sensitivity of those signals. As in knownstrain gauge configurations there may be a single resistance element ortwo or more such elements may be formed by the thin-film deposit, e.g.in a rosette pattern. The conductive deposits over the resistive elementact each as a shunt, particularly over the junction regions with theconductors, to exclude these regions from the resistance measurement andthereby eliminate stray signals.

In the formation of the strain gauge, holes are first drilled in themetallic substrate 4 to receive the conductors 6 and glass plugs 8 areinserted in these holes, the conductor leads then being pushed throughthe plugs while the glass is plastic to project slightly above thesurface on one side of the substrate. With the glass bonding and sealingthe leads in their holes, the surface of that side of the substrate isground and polished to a high degree of flatness, and the glass layer 10is deposited, conveniently by radio frequency sputtering to form aninsulating layer about 6 microns thick. Before the layer 10 isdeposited, the areas immediately over the polished ends of theconductors 6 are masked but the masking allows the glass layer 10 tooverlap the annular end-section areas of the associated glass plugs sothat a continuous insulating barrier covers the face of the substrate.

In some instances, e.g. because of a particular circuit pattern layout,it may not be practical to mask the substrate during the deposition ofthe glass layer 10. The ends of the conductors 6 can then be exposed byetching the glass layer before proceding to the next step.

For this following step, Nichrome is deposited on the surface of theglass layer 10 and the exposed ends of the conductors, also convenientlyby radio frequency sputtering to a thickness somewhat less than 1micron, and the deposit is applied through a mask or is etched afterdeposition to form one or more resistance elements. The precedingmasking of the glass layer 10 ensures that the resistance element 2makes good contact with the end faces of the conductors, although it iscompletely isolated from the metallic substrate.

A further deposit is now laid down, this time of an electricallyconductive material to provide the layer 12, preferably somewhat thickerthan the resistive material layer, and the deposition surface is eithermasked or the deposit is subsequently etched to confine the conductivematerial layer to specific regions including the areas immediately overthe discontinuities in the initial insulating glass layer 10. The finalpassivating layer 14 of glass, some 2-6 microns thick is then appliedover the resistive and conductive deposits 2,12 to encapsulate them.

With a strain gauge constructed in this manner, the advantages of usinga thin-film strain-responsive element are available without thedisadvantage of requiring wire bonds. In addition the construction iscompact and can be used in chemically active environments and atelevated temperatures far beyond the capabilities of conventional straingauges. If the strain gauge fronts onto a space containing corrosivematerials and/or at a high temperature, the conductors emerging from theopposite side of the substrate can be protected from that environment.At the same time, the conductors provide very simple and robustlead-outs for connecting the strain-sensitive element to an externalmeasuring circuit.

These features allow a design to be produced which can be used in hightemperature or chemically active environments but which retains theknown advantages of thin-film gauges in long term stability and freedomfrom creep and hysterisis.

Examples of strain gauges according to the invention formed in themanner described above are shown in FIGS. 3 to 5.

In FIG. 3, a gauge is shown arranged to be used as a cantilever.Projecting from a support 30, the load L applied near its free endplaces the upper face of the cantilevered substrate or body 34 intension, the magnitude of which determines the resistance of thethin-film circuit element 32.

FIG. 4, shows a gauge arranged to measure the pressure P acting on thetop face of a diaphragm-like substrate or body 44, for which purpose anarea 44a of the body underlying the resistance element has its thicknessreduced to increase sensitivity. It will be noted that the element 42 issubjected directly to the strains of the surface exposed to the pressureP, it being protected from hostile atmospheric conditions in thepressure zone by the passivating layer 52, while the disposition of thelead-outs 46 both protects them from these conditions and ensures thatthe connection of the circuit element to an external measurement circuit(not shown) can be made with no risk of establishing a fluid leakagepath through the diaphragm body.

FIG. 5, illustrates a strain gauge and with a plug-form body 54 havingthe general configuration described in GB Pat. No. 2 050 624A. It showshow the lead-outs 56, can be arranged to provide the connections to theexternal measurement circuit (not shown) while forming an integral partof the plug insert in a bore in the surface of the member 50 beingmonitored.

I claim:
 1. An electrical strain gauge comprising a metallic substratehaving an essentially smooth surface, and including a plurality ofapertures extending through the substrate to said surface, electricallynon-conductive plugs bonded into said apertures, conducting leadsextending through said plugs so as to be isolated from the substrate andprojecting to said surface of the substrate, said leads having endsterminating flush with said surface, an electrically non-conductivelayer bonded to said surface, said non-conductive layer having at leastone discontinuity therein at the location of said ends of said leadsprojecting to said surface, a thin-film deposit bonded to saidnon-conductive layer forming a strain-responsive electrical circuitelement insulated by said non-conductive layer from the metallicsubstrate, portions of said thin-film deposit forming said circuitelement overlying said at least one discontinuity of the non-conductivelayer so as to establish direct electrical contact between the leads andthe thin-film deposit forming said circuit element bonded to saidnon-conductive layer, a deposit of conductive material in local areasoverlying, and in direct contact with, respective portions of thethin-film deposit circuit element which overlay said ends of said leads,and a protective layer overlying and encapsulating the thin-film depositcircuit element.
 2. An electrical strain gauge comprising a metallicsubstrate, an essentially smooth surface on said substrate, anelectrically non-conductive layer bonded to said surface, a thin-filmdeposit bonded to said layer forming a strain-responsive electricalcircuit element, said thin-film deposit being insulated by said layerfrom the metallic substrate, and said metallic substrate including aplurality of apertures extending through the substrate to said surface,conducting leads extending through said plugs so as to be isolated fromthe substrate and projecting to said surface of the substrate, saidleads having substantially smooth ends terminating flush with saidsurface, said non-conductive layer having at least one discontinuity atthe location of said ends, and said thin-film deposit including portionsoverlaying said at least one discontinuity so as to establish electricalcontact between the leads and said thin-film deposit bonded to saidnon-conductive layer, and a protective layer over said thin-film depositso as to encapsulate said thin-film deposit.
 3. A strain gauge accordingto claim 2 wherein the substrate has a face opposite to said surface,the aperture in said substrate extending to said opposite face and theconducting leads projecting from said face for connection to an externalcircuit.
 4. A strain gauge according to claim 2 wherein a deposit ofconductive material is provided in local areas in direct contact withrespective portions of the circuit element overlying said lead ends. 5.A strain gauge according to claim 2 wherein at least one layer of saidelectrically insulating layer and protective layer is composed ofglasses.
 6. A strain gauge according to claim 2 wherein the insulatinglayer is formed as a thin-film deposit not substantially more than sixmicrons thick.